The Flow Of Gases In Furnaces

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242 APPENDIX VIIJ
a draft depression below the regenerators sufficient to remove
the-products of combustion from the furnace; additional draft
depression must be supplied to overcome any friction in the pas-
sages through which the waste gases pass. There are limits to
the height of chimney which it is desirable to install in connection
with an open-hearth furnace and any further increase in the draft
depression must be secured by the installation of a draft fan.
When these fans are operated by steam they will consume about
one-fifth of the total steam generated and the balance will be
available for the gas producers and for supplying the other power
demands of the plant.
The primary function of the open-hearth furnace is the produc-
tion of steel and this should be kept in mind in the design of the
waste-heat boiler setting and flues. The boiler should be by-
passed so that any failure in these portions of the equipment will
not necessitate the shut-down of the furnace. In some cases
steam-jet apparatus has been installed to provide against the
failure of the fan or its motive power.
The venturi coned ejector form of chimney has been employed
in some installations. This type of chimney may be used in either
of two methods, the fan may be reduced in size and only handle
a portion of the waste gases, or the Louis Prat method may be
employed, in which the fan handles cold air only. This latter
method is analogous to that of the hydraulic head increaser
designed by Clemens Herschel for use with low head hydro-
plants.
One of the incidental advantages of introducing induced draft-
in the operation of the furnace arises from the fact that the fan
draft may be increased to compensate for the blocking up of the
checkers and the operation of the furnace will be entirely inde-
pendent of those barometric and weather conditions which affect
chimney draft.
Reversing valves for regeneratively fired furnaces have been a
source of much trouble. Many different valves have been
designed and placed upon the market, and a number of different
flue arrangements have been devised to eliminate the reversing
valve and accomplish the reversal with a multiplicity of mushroom
valves and dampers. The simple Siemens butterfly valve was the
first four-way valve used on these furnaces. When in use, how-
ever, and exposed, on one side, to the hot gases and on the other,